A variety of processes are used to recover viscous hydrocarbons, such as heavy crude oils and bitumen, from underground deposits. There are extensive deposits of viscous hydrocarbons throughout the globe, including large deposits in the Northern Alberta tar sands, that are not recoverable with traditional oil well production technologies. A problem associated with producing hydrocarbons from such deposits is that the hydrocarbons are too viscous to flow at commercially viable rates at the temperatures and pressures present in the reservoir. In some cases, these deposits are mined using open-pit mining techniques to extract the hydrocarbon-bearing material for later processing to extract the hydrocarbons.
Alternatively, thermal techniques may be used to heat the reservoir fluids and rock to produce the heated, mobilized hydrocarbons from wells. One such technique for utilizing a single well for injecting heated fluids and producing hydrocarbons is described in U.S. Pat. No. 4,116,275, which also describes some of the problems associated with the production of mobilized viscous hydrocarbons from horizontal wells.
One thermal method of recovering viscous hydrocarbons using two vertically spaced wells is known as steam-assisted gravity drainage (SAGD) process. The SAGD process is currently the only commercial process that allows for the extraction of bitumen at depths too deep to be strip-mined. For example, the estimated amount of bitumen that is available to be extracted via SAGD constitutes approximately 80% of the 1.3 trillion barrels of bitumen in place in the Athabasca oilsands in Alberta, Canada. Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 and corresponding U.S. Pat. No. 4,344,485. In the SAGD process, steam is pumped through an upper, horizontal injection well into a viscous hydrocarbon reservoir while the heated, mobilized hydrocarbons are produced from a lower, parallel, horizontal production well vertically spaced proximate to the injection well. The injection and production wells are typically located close to the bottom of the hydrocarbon deposits.
The SAGD process is believed to work as follows. The injected steam creates a “steam chamber” in the reservoir around and above the horizontal injection well. As the steam chamber expands upwardly and laterally from the injection well, viscous hydrocarbons in the reservoir are heated and mobilized, especially at the margins of the steam chamber where the steam condenses and heats a layer of viscous hydrocarbons by thermal conduction. The heated, mobilized hydrocarbons (and steam condensate) drain under the effects of gravity towards the bottom of the steam chamber, where the production well is located. The mobilized hydrocarbons are collected and produced from the production well. The rate of steam injection and the rate of hydrocarbon production may be modulated to control the growth of the steam chamber to ensure that the production well remains located at the bottom of the steam chamber and in a position to collect the mobilized hydrocarbons.
In order to initiate a SAGD production, thermal communication must be established between an injection and a production SAGD well pair. Initially, the steam injected into the injection well of the SAGD well pair will not have any effect on the production well until at least some thermal communication is established because the hydrocarbon deposits are so viscous and have little mobility. Accordingly, a start-up phase is required for the SAGD operation. Typically, the start-up phase takes about three months before thermal communication is established between the SAGD well pair, depending on the formation lithology and the actual inter-well spacing.
The traditional approach to starting-up the SAGD process is to simultaneously operate the injection and production wells independently of one another to circulate steam. The injection and production wells are each completed with a screened (porous) casing (or liner) and an internal tubing string extending to the end of the liner, forming an annulus between the tubing string and casing. High pressure steam is simultaneously injected through the tubing string of both the injection and production wells. Fluid is simultaneously produced from each of the injection and production wells through the annulus between the tubing string and the casing. In effect, heated fluid is independently circulated in each of the injection and production wells during the start-up phase, heating the hydrocarbon formation around each well by thermal conduction. Independent circulation of the wells is continued until efficient thermal communication between the wells is established. In this way, an increase in the fluid transmissibility through the inter-well span between the injection and production wells is established by conductive heating. The pre-heating stage typically takes about three to four months. Once sufficient thermal communication is established between the injection wells, the upper, injection well is dedicated to steam injection and the lower, production well is dedicated to fluid production. Canadian Patent No. 1,304,287 teaches that in a SAGD start-up process, while the injection and production wells are being operated independently to inject steam, the steam must be injected through the tubing string and fluid collected through the annulus, not the other way around. The patent discloses that if steam is injected through the annulus and fluid collected through the tubing string, the steam looses heat to both the formation and the tubing string (and its contents), causing the injected steam to condense before reaching the end of the well.
U.S. Pat. No. 5,215,146 describes a method for reducing start-up time in SAGD operation by maintaining a pressure gradient between the upper and lower wells with foam. The pressure gradient forces the hot fluids from the upper well to the lower well. However, the method adds undesired costs and maintenance requirements due to the need to create downhole foam which is typically not required in a SAGD process.
WO 99/67503 teaches a method for initiating the recovery of hydrocarbons by injecting heated fluids into the hydrocarbon deposit through an injection well while withdrawing fluids from a production well. The flow of the heated fluid between the injection and the production wells warms the reservoir fluids and rock between the wells to establish suitable conditions for recovery of hydrocarbons. However, the method adds undesired costs and maintenance requirements due to the need to inject heated fluids which are not typically required in a SAGD process.
Accordingly, an accelerated start-up method is needed to decrease the start-up time for SAGD operation that does not require the injection of heated fluids or the creation of downhole foam. Further, such a start-up method should accelerate start-up of SAGD operations without adversely impacting production from the SAGD well pair.